The present invention pertains to broadband intermediate frequency (IF) conversion using two analog to digital converters (ADCs), and to approaches for correcting differences between the two ADCs.
In broadband receivers, the conversion of broadband intermediate frequency (IF) signals to digital samples has been approached in several ways. A simple approach is to use a single high-speed analog to digital converter (ADC) to convert the analog IF signal to digital samples directly. For this approach to work, the ADC sample rate Fs must be greater than twice the bandwidth of the IF signal. The inherent trade-off between ADC sample rate and accuracy has limited the ability of this approach to achieve sufficient accuracy for broadband IFs.
Another common approach is to convert the IF signal to baseband In-phase and Quadrature (I/Q) signals by using a pair of mixers in a co-quad configuration. The I and Q signals are then digitized by a pair of ADCs. The sample rate of these ADCs must be greater than the original bandwidth of the original IF signal. This sample rate is half the sample rate required for the single ADC approach. However, analog inaccuracies in the conversion from IF to I/Q signals introduces inaccuracies that limit this approach""s ability to achieve sufficient accuracy for broadband signals.
A third approach is to sample the IF directly using multiple high accuracy ADCs in a round-robin fashion to achieve a sufficiently high composite sampling rate. This approach has not been used for communications IF signals because of the requirement to achieve very precise matching and timing offsets of the multiple ADC samples. The multiple ADC approach has been used for other lower accuracy applications such as high bandwidth oscilloscopes.
What is needed is a way to calibrate and correct for mismatch between two ADCs in a two-ADC I/Q system.
By stimulating a two-ADC IF to I/Q conversion system with a known input, a mechanism for calibrating and correcting mismatch between the ADCs is provided. Mismatches in amplitude and time can be efficiently corrected in real-time ADC sampling at high sample rates by passing ADC samples through a derived finite impulse response (FIR) filter.